A conventional technique for coating cylindrical or drum shaped photoreceptor substrates involves dipping the substrates in coating baths. The bath used for preparing photoconducting layers is prepared by dispersing photoconductive pigment particles in a solvent solution of a film forming binder. Unfortunately, some organic photoconductive pigment particles cannot be applied by dip coating to form high quality photoconductive coatings. For example, organic photoconductive pigment particles such as hydroxygallium phthalocyanine pigment particles tend to settle when attempts are made to disperse the pigments in a solvent solution of a film forming binder. The tendency of the particles to settle requires constant stirring which can lead to entrapment of air bubbles that are carried over into the final photoconductive coating deposited on a photoreceptor substrate. These bubbles cause defects in final prints xerographically formed with the photoreceptor. The defects are caused by differences in discharge of the electrically charged photoreceptor between the region where the bubbles are present and where the bubbles are not present. Thus, for example, the final print will show dark areas over the bubbles during discharged area development or white spots when utilizing charged area development. Moreover, many pigment particles tend to agglomerate when attempts are made to disperse the pigments in solvent solutions of film forming binders. The pigment agglomerates lead to nouniform photoconductive coatings which in turn lead to other print defects in the final xerographic prints due to non-uniform discharge. The film forming binder selected for photoconductive pigment particles in a charge generating layer can adversely affect the particle dispersion uniformity, coating composition rheology, residual voltage after erase and electrophotographic sensitivity. Some binders can lead to unstable pigment particle dispersions which are unsuitable for coating photoreceptors. Thus, for example, when a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate such as VYHH terpolymer from Union Carbide is utilized to disperse hydroxygallium phthalocyanine photoconductive particles, an unstable dispersion is obtained. Moreover, a charge generating layer containing this copolymer has poor light sensitivity and gives high residual voltage after erase. Combinations of some polymers can result in unacceptable coating or electrical properties. For example, some polymers are incompatible with each other and cannot form coatings in which the polymers or particles are distributed uniformly throughout the final coating.
Photoconductive compositions are also difficult to modify for electrophotographic copiers, duplicators and printers characterized by different sensitivity requirements. Thus, custom photogenerating layer compositions must be prepared for each type of machine having its own different specific sensitivity requirement. The addition of a relatively insensitive pigment to a highly sensitive photoconductive pigment can alter the overall sensitivity of a photoreceptor. However, uniform electrical characteristics from one batch to the next batch is difficult to achieve because of uneven pigment distribution of the two different pigment particles in the final dried charge generation layer. Variations in distribution might be due to property differences of the different pigment materials employed such as size, shape, wetting characteristics, density, triboelectric charge, and the like. For example, some dispersions behave in a non-uniform manner when deposited as a coating on a photoreceptor substrate to form discontinuous coatings during dip coating or roll coating operations. It is believed that these discontinuous coatings are caused by the coating material flowing in some regions of the areas being coated and not in other regions. Thus, there is a need which the present invention addresses for new charge generating compositions containing two types of pigments that exhibit good dispersion and coating qualities.
Conventional charge generating compositions are disclosed in Nealey et al., U.S. Pat. No. 5,681,678; Nealey et al., U.S. Pat. No. 5,725,985; Burt et al., U.S. Pat. No. 5,456,998; and Nealey et al., U.S. Pat. No. 5,418,107.
Photoreceptors have been commercially available from Xerox Corp. for over a year that contain a layer of a charge generating composition composed of a hydroxygallium phthalocyanine, an alkoxy-bridged metallophthalocyanine dimer, and a polymer matrix ("VMCH") of 86% by weight vinyl chloride, 13% by weight vinyl acetate, and 1% by weight maleic acid where the VMCH has a molecular weight of about 27,000.